Version 5.1.24 of MySQL introduced the start-up option
innodb_adaptive_hash_index that allows the adaptive hash index
to be disabled. It is enabled by default. Starting with
InnoDB Plugin 1.0.3, the parameter can be modified by the
SET GLOBAL command, without restarting the
server. Changing the setting requires the
SUPER privilege.

Disabling the adaptive hash index will empty the hash table
immediately. Normal operations can continue while the hash table
is emptied, and executing queries that have been using the hash
table will access the index B-trees directly instead of
attempting to utilize the hash index. When the adaptive hash
index is enabled, the hash table will be populated during normal
operation.